Wireless communication systems, for example cellular telephony or private mobile radio communication systems, typically provide for radio telecommunication links to be arranged between a plurality of base transceiver stations (BTSs) and a plurality of subscriber units, often termed mobile stations (MSs).
The communication link from a BTS to a MS is generally referred to as a down-link communication channel. Conversely, the communication link from a MS to a BTS is generally referred to as an up-link communication channel.
In a wireless communication system, each BTS has associated with it a particular geographical coverage area (or cell). The coverage area is defined by a particular range where the BTS can maintain acceptable communications with MSs operating within its serving cell. Often these cells combine to produce an extensive coverage area.
Wireless communication systems are distinguished over fixed communication systems, such as the public switched telephone network (PSTN), principally in that mobile stations move between coverage areas served by different BTS (and/or different service providers) and, in doing so, encounter varying radio propagation environments.
In such wireless communication systems, methods for communicating information simultaneously exist where communication resources in a communication network are shared by a number of users. Such methods are termed multiple access techniques. A number of multiple access techniques exist, whereby a finite communication resource is divided into any number of physical parameters, such as:                (i) frequency division multiple access (FDMA) whereby frequencies used in the communication system are shared,        (ii) time division multiple access (TDMA) whereby each frequency used in the communication system, is shared amongst users by dividing the communication resource (each frequency) into a number of distinct time periods (time-slots, frames, etc.), and        (iii) code division multiple access (CDMA) whereby communication is performed by using all of the respective frequencies, in all of the time periods, and the resource is shared by allocating each communication a particular code, to differentiate desired signals from undesired signals.        
Within such multiple access techniques, different duplex (substantially simultaneous two-way communication) paths are arranged. Such paths can be arranged in a frequency division duplex (FDD) configuration, whereby a frequency is dedicated for up-link communication and a second frequency is dedicated for down-link communication. Alternatively, the paths can be arranged in a time division duplex (TDD) configuration, whereby a first time period is dedicated for up-link communication and a second time period is dedicated for down-link communication.
In the field of this invention it is known that a wireless cellular/mobile radio system's performance, capability, and coverage is highly dependent on a number of factors. For example, the subscriber data rate demand and the location of a subscriber may play a crucial role in determining the efficiency and availability of communication to/from the unit itself as well as to/from neighboring users. In addition, these factors are known to be very time-dependent, i.e. the factors vary widely over time. This is particularly the case in mixed voice and data wireless communication systems with users frequently switching between being active and inactivestates in, as well as being geographically widely dispersed within, the wireless communication system.
In the context of one-to-one or one-to-many wireless communications, for example in the field of private mobile radio (PMR), it is known that a MS may operate outside a dedicated network coverage area by communicating in a direct communication link with at least one other MS. Such a communication mode is generally referred to as either direct-mode operation (DMO) (when used in a conjunction with a system supporting trunked mode), conventional, two-way, or back-to-back communication.
Hence, when a MS operates in a direct mode, there is no system controller and therefore no centralised timing synchronisation or infrastructure-controlled power control to help set up calls, organise communication resource to minimise interference, etc.
In direct-mode, communication links are established in a similar manner as to the direct-mode operation of conventional half duplex radio schemes used by many existing private mobile radio systems such as that of the emergency services. Such calls are set up by a direct-mode (calling) communication unit selecting an address (e.g. 5-tone signalling sequence or digital selective calling) of a receiving (called) communication unit. The called communication unit then is directly contacted to set up a communication link.
Furthermore, as open communication systems are designed to share a finite communication resource, amongst a large number of subscriber devices, the subscriber devices are designed with operation and functionality limitations, such as a maximum data rate that they can transmit. Alternatively, they are designed to include all features possible, without any regard to whether the user will actually use them. This leads to inefficiency, as perceived by the user as well as with regard to the operation of the system.
In a wireless communication network, it is well known that radio transmissions are subjected to rayleigh fading, multipath and other noise-inducing effects. Therefore there is a need to provide some form of error protection and/or error correction to most transmissions. As particular transmissions require increased reliability, such as emergency calls, these transmissions are provided with increased amounts of error protection and thereby redundancy. Again, this is an inefficient use of what is always a limited resource.
In the context of linking wireless communication units, it is also known that linking cellular communication units directly together, using for example the Opportunity Driven Multiple Access (ODMA) mode proposed in UMTS (TDD mode), may provide enhanced data rates. In this mode of operation, some of the overall time-frequency resource of the system is used to enable mobiles to relay signals to other users.
As defined in 3GPP release 99 specification for UMTS TDD, OMDA can allocate a part of its resources to allow mobiles to relay information between themselves such that a mobile can ‘connect’ to a network through another mobile. Since the TDD system works on time-slots, it is possible to allocate part of the system resources for mobile to mobile communication. This type of system, can increase coverage at the cost of bandwidth.
As a consequence, in order to resolve such problems in a general sense, it would be helpful if the communication solution could be standardized. However, a major disadvantage with ODMA is that it is complex. Furthermore, ODMA requires a substantial amount of signalling resource, to facilitate the relaying of communications between the directly coupled cellular units. Therefore, due to the above difficulties with ODMA, it is unlikely that such a complex and resource-hungry solution would be acceptable in an open communication standard.
Thus, there exists a need in the field of the present invention to provide linking of multiple communication units wherein the above mentioned disadvantages may be alleviated.